I'm interested in at least learning about it, but the material seems so specialized I don't know where to start. Is it necessary to learn other things first and then start on alchemy, or is there some sort of online course you can take?

You dont say what kind of Alchemy you want to learn. Philosophic Alchemy that just uses symbols?Physical Alchemy where you assemble messy tubes and glassware to extract Philosopher's Stones of some plant, or heavy furnace and crucible to melt metals? The Adept's Alchemy that uses magical processes to create transformation? Maybe Jungian Alchemy that speaks Psychology using Alchemical symbolism?

Sorry, I guess I was referring to physical alchemy, which is the most overwhelming to me. Although I do love Jung and the symbolism of alchemy. But I can figure out psychology (tolerably) on my own. It's my college major.

From my limited understanding, is that plant alchemy has its own benefits as well as instructing one in the technical skills that are used in mineral/metallurgic alchemy without the volatility of the materials in those branches. That is why it seems that plant based alchemy is instructed first, then other branches. I'm probably oversimplifying things though : )